\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=latex,scale=1]
  \useasboundingbox (-0.1,0.1) rectangle(6,-3);
  \draw [-stealth](0,0) parabola (6,-3);
  \draw[->](0,0)--++(1.2,0)node[right]{$v$};
  \draw[->](0,0)--++(0,-0.8)node[right]{$G$};
  \draw[->](2.4,-0.48)--++(0,-0.8)node[right]{$G$};
  \draw[->](2.4,-0.48)--++(1.15,-0.46)node[above]{$v$};
\end{tikzpicture}
\end{document}